Stochastic Response Analysis of Deepwater Structures in Short-Crested Random Seas

1999 ◽  
Vol 121 (3) ◽  
pp. 181-186 ◽  
Author(s):  
P. Teigen ◽  
A. Naess
Keyword(s):  
Time Domain ◽  
Large Volume ◽  
Time Domain Analysis ◽  
Order Theory ◽  
Offshore Structures ◽  
Response Analysis ◽  
Random Waves ◽  
Stochastic Response ◽  
Cpu Time ◽  
Random Seas

The paper discusses the problem of estimating the response statistics of moored large-volume offshore structures subjected to short-crested random waves. A general second-order theory is described that makes it possible to carry out the entire analysis in the frequency domain, which is computationally more efficient than time domain analysis, which generally requires considerably more CPU time to reach the same level of accuracy.

Accurate Numerical Methods for Calculating the Response Statistics of Compliant Offshore Structures

2005 ◽  
Author(s):  
A. Naess ◽  
H. C. Karlsen ◽  
P. S. Teigen
Keyword(s):  
Numerical Methods ◽  
Volterra Series ◽  
State Of The Art ◽  
Offshore Structures ◽  
Second Order ◽  
Stochastic Response ◽  
Random Seas ◽  
The Mean ◽  
Extreme Responses ◽  
Key Parameter

The state-of-the-art representation of the horizontal motions of e.g. a TLP in random seas is in terms of a second order stochastic Volterra series. Until recently, there has been no method available for accurately calculating the mean level upcrossing rate of such response processes. Since the mean upcrossing rate is a key parameter for estimating the large and extreme responses it is clearly of importance to develop methods for its calculation. The paper describes numerical methods for calculating the mean level upcrossing rate of a stochastic response process represented as a second order stochastic Volterra series. Since no approximations are made, the only source of inaccuracy is in the numerical calculation, which can be controlled.

scholarly journals Stochastic Response of Tension Leg Platform to Wave and Current Forces

1989 ◽  
Vol 111 (4) ◽  
pp. 221-230 ◽  
Author(s):  
A. Ertas ◽  
J.-H. Lee
Keyword(s):  
Constant Current ◽  
Response Analysis ◽  
Stochastic Dynamic ◽  
Superposition Method ◽  
Random Waves ◽  
Stochastic Response ◽  
Tension Leg Platform ◽  
Single Degree Of Freedom ◽  
Simulation Techniques ◽  
Wave Induced

The linear analysis in the frequency domain is presented for the surge motion of a tension leg platform (TLP) in the case of random waves only and random waves with constant current. A single-degree-of-freedom model of a TLP is employed for response. The superposition method, one of the simulation techniques, is applied to random sea wave, and the response analysis of TLP in time is developed with wave velocity and wave acceleration simulations. Wave-induced forces are calculated using the modified Morison equation, which takes into account relative motion. Computational methods for both analyses are developed, and the results of stochastic, dynamic response of the TLP, with and without the presence of current, are presented and compared.

Response Analysis on Asymmetric Twin Tower Building with Large Bases Subjected to Seismic Action

2011 ◽  
Vol 117-119 ◽  
pp. 174-179
Author(s):  
Qiong Fen Wang ◽  
Ji Yao ◽  
Liang Cao ◽  
Jian Bin Xie ◽  
Chun Li Guo
Keyword(s):  
Time Domain ◽  
Structural Design ◽  
Time Domain Analysis ◽  
Response Analysis ◽  
Seismic Action ◽  
Analysis Method ◽  
Structural Dynamic ◽  
Fem Model ◽  
Structural Dynamic Characteristics ◽  
High Building

In this paper, asymmetric twin tower high buildings with large bases were studied and the FEM model of asymmetric twin tower high building with large bases was established. Then the structural dynamic characteristics was analyzed, the x-direction and y-direction level seismic excitation were input separately with the help of time-domain analysis method, and the acceleration time-domain curve of the top of the structure was obtained, the reference of structural design was provided.

Computation of Wave-Induced Drift Forces Introduced by Displaced Position of Compliant Offshore Platforms

1992 ◽  
Vol 114 (3) ◽  
pp. 175-184 ◽  
Author(s):  
Y. Li ◽  
A. Kareem
Keyword(s):  
Time Domain ◽  
Phase Relationship ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Offshore Structures ◽  
Time Dependent ◽  
Wave Loads ◽  
Nonlinear Feedback ◽  
The Time Domain ◽  
Drift Forces

The wave forces computed at the displaced position of offshore structures may introduce additional drift forces. This contribution is particularly significant for compliant offshore structures that are configured by design to experience large excursions under the environmental load effects, e.g., tension leg platform. In a random sea environment, this feature can be included in the time domain analysis by synthesizing drag and diffraction forces through a summation of a large number of harmonics with an appropriate phase relationship that reflects the platform displaced position. This approach is not only limited to the time domain analysis, but the superposition of a large number of trigonometric terms in such an analysis requires a considerable computational effort. This paper presents a computationally efficient procedure in both the time and frequency domains that permits inclusion of the time-dependent drift forces, introduced by the platform displacement, in terms of linear and nonlinear feedback contributions. These time-dependent feedback forces are expressed in terms of the applied wave loads by linear and quadratic transformations. It is demonstrated that the results obtained by this approach exhibit good agreement with the procedure based on the summation of trigonometric functions.

Application of Peak Distribution Method for Response Based Analysis of Mooring Lines Under Tropical Storms

2020 ◽  
Author(s):  
A. Ghasemi ◽  
Y. Drobyshevski ◽  
M. Kimiaei ◽  
M. Efthymiou
Keyword(s):  
Time Domain ◽  
Nonlinear Behavior ◽  
Extreme Value Distribution ◽  
Time Domain Analysis ◽  
System Response ◽  
Response Analysis ◽  
Tropical Storms ◽  
Mooring Lines ◽  
Distribution Method ◽  
Large Variability

Abstract Response based analysis (RBA) is a comprehensive approach for the prediction of extreme responses and design metocean conditions of offshore facilities. For RBA, the structural system needs to be modelled, and its behavior analyzed when subjected to large metocean datasets, usually comprising thousands of different sea states. Due to the dynamic and nonlinear behavior of mooring systems in floating structures, application of conventional time domain analysis for RBA of these systems is a computationally demanding process. Hence, investigation of faster solvers and more efficient methods for the RBA is inevitable. Peak distribution method (PDM), which has recently been introduced and used for response analysis of mooring systems under extreme design conditions, is a possible solution to reduce the computational efforts in RBA by reducing the number of simulations. This study explores the utilization of the PDM for RBA of the mooring system of a turret-moored large FPSO subjected to tropical storms. Large variability of metocean parameters within such storms limits the applicability of intuitive judgement for the selection of governing sea states. The results are compared through both time-domain and frequency-domain simulations and a computationally efficient methodology is proposed. It provides a general robust framework of computing the extreme value distribution of the system response. The proposed methodology can be used for RBA of mooring lines tension under storm conditions comprising large number of sea states.

Non-Linear Ship Rolling in Random Seas by the Statistical Non-Linearization Technique

1995 ◽  
Author(s):  
C. W. S. To
Keyword(s):  
Averaging Method ◽  
Stochastic Averaging ◽  
Nonlinear Damping ◽  
Offshore Structures ◽  
Stochastic Averaging Method ◽  
Random Waves ◽  
Linearization Technique ◽  
Random Seas ◽  
Ship Safety ◽  
Ship Rolling

Abstract The study of motion of ships or similar slender offshore structures in irregular seas has, in recent years, gathered sufficient interest due to the fact that experimental data and various approximate techniques are readily available, and that ship safety is a much more expensive issue. Central to the study is the problem of predicting ship rolling motion due to random waves. In this paper a method, employing the statistical non-linearization technique that was developed by the author, is applied to evaluate the joint stationary probability density of response of a model proposed by Roberts and associates for the analysis of nonlinear ship rolling in random seas. For small nonlinear damping and small intensity of excitation the result is identical to that obtained with the generalized stochastic averaging method of energy envelope presented earlier by the author.

Environmental Load Effect Analysis of Guyed Towers

1985 ◽  
Vol 107 (1) ◽  
pp. 24-33 ◽  
Author(s):  
O. Mo ◽  
T. Moan
Keyword(s):  
Time Domain ◽  
Structural Model ◽  
Dimensional Space ◽  
Time Integration ◽  
Offshore Structures ◽  
Random Waves ◽  
Significant Discrepancy ◽  
Effect Analysis ◽  
Load Effect ◽  
Fluid Field

A general method for dynamic load effect analysis of slender offshore structures subjected to short crested random waves, current and wind, is given. The structure is represented by a three-dimensional space frame model utilizing dash-pots and linear or nonlinear spring elements to represent guy lines and coupling between structure and foundation. The component mode synthesis formulation is adopted for reduction of the number of degrees of freedom. The hydrodynamic forces are computed by Morison’s equation, accounting for finite wave elevation, directionality, and relative fluid-structure motion. Various kinematic models for the fluid field in the splashing zone are compared. To get a reasonable representation of nonlinearities in the loading and the structural model, a Monte Carlo approach is adopted. Starting with simulated samples of the random fluid field and wind forces, time series of structural responses are found by numerical time integration utilizing the Newmark β-family of time integration operators. Numerical results for a guyed tower at 450-m water depth are presented. The statistical uncertainties associated with the stochastic time domain simulations are discussed. A significant discrepancy is found between linearized frequency domain solutions and the present nonlinear time domain formulation. The importance of an adequate representation of superharmonic responses is particularly discussed. The differences in results due to various solution methods are found to vary significantly with sea-state conditions.

A Preliminary Study of a Rigid Semi-Submersible Fish Farm for Open Seas

2017 ◽  
Author(s):  
Lin Li ◽  
Muk Chen Ong
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Time Domain Analysis ◽  
Fish Farm ◽  
Response Analysis ◽  
Frequency Domain Method ◽  
Mooring System ◽  
Viscous Effects ◽  
Support Structure ◽  
Mooring Lines

The development of reliable fish farm structures for open seas becomes more and more important. One of the challenges is to design a robust structure to withstand the harsh offshore environmental loads. This paper investigates a semi-submersible type offshore fish farm system for open seas. This system consists of a semi-submersible support structure with pontoons and braces, a catenary mooring system and net cages. The support structure is designed to be rigid to resist severe offshore conditions. A preliminary hydrodynamic and response analysis is carried out for this concept. The linear hydrodynamic properties using different composite models with panel and Morison elements are computed. Based on the hydrodynamic analysis, linearised frequency-domain and coupled time-domain analysis are performed to predict the extreme motions of the support structure and the extreme tensions in the mooring lines. The results indicate that the frequency-domain method underestimates the extreme responses, and the couplings between the structure and the mooring system need to be considered in the time-domain. Responses using various hydrodynamic models are also compared to evaluate the influences of the viscous effects from the pontoons and the nets of this fish farm concept.

Time-Domain Analysis of Floating Bodies with Forward Speed

2002 ◽  
Vol 124 (2) ◽  
pp. 66-73 ◽  
Author(s):  
Gu¨nther F. Clauss ◽  
Katja Stutz
Keyword(s):  
Nonlinear Wave ◽  
Time Domain ◽  
Degrees Of Freedom ◽  
Wave Structure ◽  
Nonlinear Effects ◽  
Time Domain Analysis ◽  
Offshore Structures ◽  
Driving Mechanism ◽  
Forward Speed ◽  
Transient Wave

Broaching, surf-riding, and capsizing of ships and offshore structures are transient wave-structure interactions which imply high risks for crew, vessel and cargo. As nonlinear effects are of great importance, time-domain investigations are indispensable. For unveiling the associated driving mechanism of these critical motions, it is desirable to analyze the cause-reaction chains in detail: Depending on the transient wave elevation, we obtain an instationary pressure distribution on the wetted surface of the cruising vessel. Resulting forces and moments excite vessel motions in six degrees of freedom. Based on the linear panel-method program for transient wave-body interactions, TiMIT [Korsmeyer et al. (1999)], this paper investigates seakeeping characteristics of offshore structures with forward speed. Results are presented in frequency and time domain. The procedure allows to identify critical seaways, and to analyze cause-reaction chains in deterministic wave sequences where critical and steep wave packets are embedded in random seas. The detailed evaluation reveals that large roll and pitch motions are easily reduced by variation of course and speed. For investigating the mechanism of wave/structure interactions, this paper introduces the relevant time-domain methodology, and indicates how nonlinear wave characteristics can be introduced in the time-stepping analysis. In subsequent steps nonlinear wave/structure interactions will also be considered.

Seismic Analysis of Intake Tunnel for Nuclear Power Plant

2011 ◽  
Vol 243-249 ◽  
pp. 3513-3517
Author(s):  
Jie Zhao ◽  
Yang Zheng ◽  
Gui Xuan Wang
Keyword(s):  
Power Plant ◽  
Nuclear Power Plant ◽  
Time Domain ◽  
Nuclear Power ◽  
Seismic Analysis ◽  
Time Domain Analysis ◽  
Internal Force ◽  
Response Analysis ◽  
Plant Structure ◽  
Seismic Response Analysis

Aiming at the nuclear power plant structure and adopting time domain analysis approach, the seismic analysis of an intake tunnel for nuclear power plant is performed with FLAC3D in this paper. Contraposing the characters of the field rock of the nuclear plant, the internal force distribution of the tunnel under different wall rocks is researched and analysed, and the envelope diagram of inner force in the lining of the tunnel is drawn. The obtained law can provide the basis for the seismic response analysis of tunnels.

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